It was a grip texture that Glock tried about 5-6 years ago.
Most found it to rough and Glock discontinued it.
Gen 1 smooth textured frame.
Gen 2 & 3 have the RTF (rough textured frame)
Gen 3 Glocks can be had with the RTF2 (most aggressive texture)
Gen 4 Glocks have the RTF3, a rough textured frame in between the RTF and RTF2.